Blood glucose concentration (blood glucose level) is an important marker for diabetes. Devices for self-monitoring of blood glucose levels by diabetes patients include widely used devices for Self-Monitoring of Blood Glucose (SMBG) that employ electrochemical biosensors. The biosensors used in SMBG devices of the prior art employ enzymes whose substrate is glucose, such as glucose oxidase (GOD). However, because GOD uses oxygen as an electron acceptor, the dissolved oxygen in measuring samples can affect measurement with SMBG devices employing GOD, potentially interfering with precise measurement values.
On the other hand, various glucose dehydrogenases (hereunder, GDH) are known as enzymes whose substrate is glucose but do not use oxygen as an electron acceptor. Specifically, there have been discovered GDH(NAD(P)-GDH) types, which use nicotinamide dinucleotide (NAD) or nicotinamide dinucleotide phosphoric acid (NADP) as coenzymes, and GDH(PQQ-GDH), which uses pyrroloquinoline quinone (PQQ) as a coenzyme, and these are employed in the biosensors of SMBG devices. However, NAD(P)-GDH has poor enzyme stability and requires addition of coenzyme, while PQQ-GDH has low substrate specificity and also acts on saccharide compounds such as maltose, D-galactose and D-xylose in addition to the measurement target glucose, and therefore saccharide compounds other than glucose in the measuring sample can affect the measured value, making it impossible to obtain an accurate measured value.
When blood glucose levels of diabetes patients that have undergone infusion are measured using SMBG devices employing PQQ-GDH as a biosensor, which have come into use in recent years, the PQQ-GDH also acts on maltose in the infusion, yielding a measured value that is higher than the actual blood glucose level, and cases have been reported in which patients have suffered hypoglycemia as a result of intervention based on such values. It has been shown that the same problem can also occur for patients undergoing galactose load testing and xylose absorption testing (see Non-patent document 1, for example). In light of this, the Pharmaceutical and Food Safety Bureau of the Ministry of Health, Labour and Welfare has conducted crossreactive testing, with the aim of examining effects on blood glucose measurement by addition of different saccharides to glucose solutions, and as a result it has been shown that addition of 600 mg/dL of maltose, 300 mg/dL of D-galactose or 200 mg/dL of D-xylose causes the measured value in a blood glucose measuring kit based on PQQ-GDH to be approximately 2.5-3 times higher than the actual glucose level. In other words, it has been determined that maltose. D-galactose and D-xylose potentially present in measuring samples result in inaccurate measured values, and it is therefore highly desirable to develop GDH with high substrate specificity allowing specific measurement of glucose without being affected by saccharide compounds that can cause measurement errors.
In light of this background, research is being conducted on types of GDH that utilize other coenzymes. For example, Non-patent documents 2 to 5 have reported GDH enzymes derived from Aspergillus oryzae, although no details are mentioned regarding their substrate specificities. Patent documents 1 to 3 disclose glucose dehydrogenase (FAD-GDH) having Aspergillus-derived flavin adenine dinucleotide (FAD) as a coenzyme, and Patent document 4 discloses Aspergillus-derived FAD-GDH with reduced activity for D-xylose.
Patent documents 1 to 4 describe FAD-GDH having low reactivity for one or more types of saccharide compounds that are not D-glucose, but no flavin-binding GDH with sufficiently low reactivity for maltose, D-galactose and D-xylose is known. Furthermore, no flavin-binding GDH is known that allows accurate measurement of glucose levels without being affected by the aforementioned saccharide compounds under conditions in which D-glucose, maltose, D-galactose and D-xylose are present.    [Patent document 1] Japanese Unexamined Patent Application Publication No. 2007-289148    [Patent document 2] International Patent Publication No. WO 04/058958    [Patent document 3] International Patent Publication No. WO 07/139,013    [Patent document 4] Japanese Unexamined Patent Application Publication No. 2008-237210    [Non-patent document 1] Pharmaceuticals and Medical Devices Safety Information No. 206, October 2004, Pharmaceutical and Food Safety Bureau of the Ministry of Health, Labour and Welfare    [Non-patent document 2] Studies on the glucose dehydrogenase of Aspergillus oryzae. I. Induction of its synthesis by p-benzoquinone and hydroquinone, T. C. Bak, and R. Sato, Biochim. Biophys. Acta, 139, 265-276 (1967)    [Non-patent document 3] Studies on the glucose dehydrogenase of Aspergillus oryzae. II. Purification and physical and chemical properties, T. C. Bak, Biochim, Biophys. Acta, 139, 277-293 (1967).    [Non-patent document 4] Studies on the glucose dehydrogenase of Aspergillus oryzae. III. General enzymatic properties, T. C. Bak, Biochim. Biophys. Acta, 146, 317-327 (1967).    [Non-patent document 5] Studies on the glucose dehydrogenase of Aspergillus oryzae. IV. Histidyl residue as an active site, T. C. Bak, and R. Sato, Biochim Biophys. Acta, 146, 328-335 (1967).